Early life adversity shapes social subordination and cell type-specific transcriptomic patterning in the ventral hippocampus.

Adverse events in early life can modulate the response to additional stressors later in life and increase the risk of developing psychiatric disorders. The underlying molecular mechanisms responsible for these effects remain unclear. Here, we uncover that early life adversity (ELA) in mice leads to social subordination. Using single-cell RNA sequencing (scRNA-seq), we identified cell type-specific changes in the transcriptional state of glutamatergic and GABAergic neurons in the ventral hippocampus of ELA mice after exposure to acute social stress in adulthood. These findings were reflected by an alteration in excitatory and inhibitory synaptic transmission induced by ELA in response to acute social stress. Finally, enhancing the inhibitory network function through transient diazepam treatment during an early developmental sensitive period reversed the ELA-induced social subordination. Collectively, this study significantly advances our understanding of the molecular, physiological, and behavioral alterations induced by ELA, uncovering a previously unknown cell type-specific vulnerability to ELA.

Myo-Inositol Levels in the Dorsal Hippocampus Serve as Glial Prognostic Marker of Mild Cognitive Impairment in Mice.

Dementia is a devastating age-related disorder. Its therapy would largely benefit from the identification of susceptible subjects at early, prodromal stages of the disease. To search for such prognostic markers of cognitive impairment, we studied spatial navigation in male BALBc vs. B6N mice in combination with in vivo magnetic resonance spectroscopy (1H-MRS). BALBc mice consistently showed higher escape latencies than B6N mice, both in the Water Cross Maze (WCM) and the Morris water maze (MWM). These performance deficits coincided with higher levels of myo-inositol (mIns) in the dorsal hippocampus before and after training. Subsequent biochemical analyses of hippocampal specimens by capillary immunodetection and liquid chromatography mass spectrometry-based (LC/MS) metabolomics revealed a higher abundance of glial markers (IBA-1, S100B, and GFAP) as well as distinct alterations in metabolites including a decrease in vitamins (pantothenic acid and nicotinamide), neurotransmitters (acetylcholine), their metabolites (glutamine), and acetyl-L-carnitine. Supplementation of low abundant acetyl-L-carnitine via the drinking water, however, failed to revert the behavioral deficits shown by BALBc mice. Based on our data we suggest (i) BALBc mice as an animal model and (ii) hippocampal mIns levels as a prognostic marker of mild cognitive impairment (MCI), due to (iii) local changes in microglia and astrocyte activity, which may (iv) result in decreased concentrations of promnesic molecules.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI).

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn2+ enters active neurons via voltage-gated calcium channels and increases local contrast in T1-weighted images. Given the property of Mn2+ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn2+ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn2+ transport. To this end, we injected mice with low dose MnCl2 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl2 injections. We assessed changes in T1 contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn2+ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn2+ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn2+ transport. The description of this critical property of Mn2+ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Mn2+ dynamics in manganese-enhanced MRI (MEMRI): Cav1.2 channel-mediated uptake and preferential accumulation in projection terminals.

Manganese-enhanced magnetic resonance imaging (MEMRI) exploits the biophysical similarity of Ca2+ and Mn2+ to map the brain's activity in vivo. However, to what extent different Ca2+ channels contribute to the enhanced signal that MEMRI provides and how Mn2+ dynamics influence Mn2+ brain accumulation after systemic administration of MnCl2 are not yet fully understood. Here, we demonstrate that mice lacking the L-type Ca2+ channel 1.2 (Cav1.2) in the CNS show approximately 50% less increase in MEMRI contrast after repeated systemic MnCl2 injections, as compared to control mice. In contrast, genetic deletion of L-type Ca2+ channel 1.3 (Cav1.3) did not reduce signal. Brain structure- or cell type-specific deletion of Cav1.2 in combination with voxel-wise MEMRI analysis revealed a preferential accumulation of Mn2+ in projection terminals, which was confirmed by local MnCl2 administration to defined brain areas. Taken together, we provide unequivocal evidence that Cav1.2 represents an important channel for neuronal Mn2+ influx after systemic injections. We also show that after neuronal uptake, Mn2+ preferentially accumulates in projection terminals.

Calcineurin inhibition blocks within-, but not between-session fear extinction in mice.

Memory extinction involves the formation of a new associative memory that inhibits a previously conditioned association. Nonetheless, it could also depend on weakening of the original memory trace if extinction is assumed to have multiple components. The phosphatase calcineurin (CaN) has been described as being involved in extinction but not in the initial consolidation of fear learning. With this in mind, we set to study whether CaN could have different roles in distinct components of extinction. Systemic treatment with the CaN inhibitors cyclosporin A (CsA) or FK-506, as well as i.c.v. administration of CsA, blocked within-session, but not between-session extinction or initial learning of contextual fear conditioning. Similar effects were found in multiple-session extinction of contextual fear conditioning and in auditory fear conditioning, indicating that CaN is involved in different types of short-term extinction. Meanwhile, inhibition of protein synthesis by cycloheximide (CHX) treatment did not affect within-session extinction, but disrupted fear acquisition and slightly impaired between-session extinction. Our results point to a dissociation of within- and between-session extinction of fear conditioning, with the former being more dependent on CaN activity and the latter on protein synthesis. Moreover, the modulation of within-session extinction did not affect between-session extinction, suggesting that these components are at least partially independent.

Memory labilization in reconsolidation and extinction--evidence for a common plasticity system?

Reconsolidation and extinction are two processes occurring upon memory retrieval that have received great attention in memory research over the last decade, partly due to their purported potential in the treatment of anxiety disorders. Due to their opposite behavioral effects, the two phenomena have usually been considered as separate entities, with few attempts to build a unified view of how both could be produced by similar mechanisms. Based on computational modeling, we have previously proposed that reconsolidation and extinction are behavioral outcomes of the same set of plasticity systems, albeit working at different synapses. One of these systems seems to be pharmacologically similar to the one involved in initial memory consolidation, and likely involves traditional Hebbian plasticity, while the second seems to be more involved with the labilization of existing memories and/or synaptic changes. In this article, we review the evidence for the existence of a plasticity system specifically involved in memory labilization, as well as its possible molecular requirements, anatomical substrates, synaptic mechanisms and physiological roles. Based on these data, we propose that the field of memory updating might ultimately benefit from a paradigm shift in which reconsolidation and extinction are viewed not as separate processes but as different instantiations of plasticity systems responsible for reinforcement and labilization of synaptic changes.

Age-dependent relevance of endogenous 5-lipoxygenase derivatives in anxiety-like behavior in mice.

When 5-lipoxygenase (5-LO) is inhibited, roughly half of the CNS effect of the prototypic endocannabinoid anandamide (AEA) is lost. Therefore, we decided to investigate whether inhibiting this enzyme would influence physiological functions classically described as being under control of the endocannabinoid system. Although 5-LO inhibition by MK-886 reduced lipoxin A4 levels in the brain, no effect was found in the elevated plus maze (EPM), even at the highest possible doses, via i.p. (10 mg/kg,) or i.c.v. (500 pmol/2 µl) routes. Accordingly, no alterations in anxiety-like behavior in the EPM test were observed in 5-LO KO mice. Interestingly, aged mice, which show reduced circulating lipoxin A4 levels, were sensitive to MK-886, displaying an anxiogenic-like state in response to treatment. Moreover, exogenous lipoxin A4 induced an anxiolytic-like profile in the EPM test. Our findings are in line with other reports showing no difference between FLAP KO or 5-LO KO and their control strains in adult mice, but increased anxiety-like behavior in aged mice. We also show for the first time that lipoxin A4 affects mouse behavior. In conclusion, we propose an age-dependent relevancy of endogenous 5-LO derivatives in the modulation of anxiety-like behavior, in addition to a potential for exogenous lipoxin A4 in producing an anxiolytic-like state.